The combustion waste gases (i.e. the exhaust) of thermal power plants, factories, on-road vehicles, diesel generators, and the like contain SOx and NOx. State and federal regulations limit the permissible amounts of these emissions because they create environment problems, such as acid rain. Accordingly, there is a continual need for improvements in the cost effective and efficient control of these emissions.
One mechanism for limiting NOx and SOx emissions is to remove or scrub the pollutants from the exhaust gas using an absorption bed, trap or similar device. Because many NOx traps have been found to be poisoned by the presence of SOx, it is important to remove as much SOx from the exhaust gas as possible. However, as compared to the large volume of studies on NOx reduction, sulfur oxide removal using solid adsorbents is an area in need of scientific advancement. For example, certain types of materials have been identified as possible solid absorbents for use in a SOx adsorption bed or traps, for example calcium oxide and alkalized alumina (Na/Al2O3 or K/Al2O3), copper-based adsorbents, e.g. Cu/Al2O3, promoted metal oxides, e.g. TiO2, Al2O3, ZrO2, promoted cerium oxide (La- or Cu-doped CeO2), and supported cobalt (Co/Al2O3). Unfortunately, over the temperature range of about 250° C. to 475° C., these materials typically have a relatively low absorption capacity. For example, their total adsorption capacity of SO2 is typically less than about 10 wt % based on the weight of the absorbent, and their breakthrough absorption capacity can be substantially lower, depending on operating conditions. As it is combustion in this temperature range that leads to a significant portion of the total SOx emissions, a greater adsorption capacity at these temperatures is needed.
One approach to increasing the absorption capacity of SOx absorption beds is to provide an oxidation catalyst upstream or admixed with the bed so as to convert most of the SO2 to SO3, since SO3 is generally more readily adsorbed than SO2 due to its formation of stable surface sulfates. However, the cost of recovery of the oxidation catalyst (frequency a precious metal) and the relatively poor conversion efficiency of SO2 to SO3 at temperatures below about 300° C. limits the effectiveness of this approach as well.
Accordingly, there is a need for solid SOx absorbents with high absorption capacity at lower temperatures and which reduce or eliminate the need for separate oxidation catalysts. In one aspect the present invention addresses this need and provides a major improvement to SOx absorption for emissions control.